The thermal stencil master plate used in stencil printing generally has a structure obtained by laminating a thermo-plastic resin film and a porous support typically consisting of a sheet of fibers. The methods for processing such a stencil master plate by perforation include the method of radiating a light beam including infrared light upon a thermal stencil master plate which is closely placed over an original, and the method of contacting a heat emitting device such as a thermal head onto a stencil master plate to form an image with a dot matrix.
According to the method of radiating a light beam including infrared light, the thermal energy absorbed by the original image causes the perforation of the thermo-plastic resin film as an optical image through an analog process, and an image identical to the original image can be formed on the stencil master plate. However, since the perforations formed in the thermo-plastic resin film of the thermal stencil master plate tend to be excessively large, deposition of ink onto the printing paper tends to be excessive. This slows down the drying of the printing ink on the printing paper, and this causes offsetting, or smearing the back of the paper due to the wet ink when the paper is piled up into a stack following the process of printing, particularly in the case of the process of rotary printing. This offsetting is particularly severe in the case of solid picture images.
According to the thermal plate making method using a thermal head, a digital process of perforation is carried out on a thermo-plastic resin film in the manner of a dot matrix by selectively heating the heat emitting elements so that a master plate image may be obtained by appropriate size distribution of the perforated dots. However, in this case, depending on the resolving power of the thermal head, the size of each of the heat emitting elements, the orientation of the fibers of the porous support, and the size of the gaps between the fibers, the perforated dots may expand and the adjacent perforated dots may merge with each other although this tendency is not so severe as in the case of the thermal plate making method based on contact duplication, and offsetting also tends to occur.
Specifically, according to the thermal plate making method using a thermal head, picture images are formed in the master plate by appropriate size distribution of perforated dots, and, in the case of a solid image, the thermal influences between adjacent heat emitting elements tends to cause excessive shrinking of the thermo-plastic resin film which in turn causes insufficiency in the cooling and solidifying of the peripheral parts of the perforated dots. As a result, lumps of the film melted during the process of perforation or parts of such lumps tend to entangle with the adjacent fibers, and this prevents passage of printing ink through the fibers during the process of printing. Further, in the printed image, because the ink which has passed through the perforations in the thermo-plastic resin film tends to contact the ink which has passed through the adjacent perforations and is about to reach or has reached the printing paper, an excessive amount of ink is often deposited on the printing paper.
Thus, according to such conventional methods, it is extremely difficult to prevent offsetting, particularly in regards to the regions of solid picture images.
To the end of solving such problems, a proposal has been made in Japanese patent laid open publication No. 02-155739 to specify the length of each of the heat emitting elements of the thermal head in the secondary scanning direction and the consistency of printing ink in a certain way. However, according to this proposal, depending on the kind of the thermo-plastic film of the thermal stencil master plate, and the ambient temperature, the size of the perforation dots and the consistency of ink tend to vary so much that the amount of ink deposition for each perforation dot becomes uneven, causing instability in the control of offsetting.